The blinking spotlight of attention

Abstract

Increasing evidence suggests that attention can concurrently select multiple locations; yet it is not clear whether this ability relies on continuous allocation of attention to the different targets (a ''parallel'' strategy) or whether attention switches rapidly between the targets (a periodic ''sampling'' strategy). Here, we propose a method to distinguish between these two alternatives. The human psychometric function for detection of a single target as a function of its duration can be used to predict the corresponding function for two or more attended targets. Importantly, the predicted curves differ, depending on whether a parallel or sampling strategy is assumed. For a challenging detection task, we found that human performance was best reflected by a sampling model, indicating that multiple items of interest were processed in series at a rate of approximately seven items per second. Surprisingly, the data suggested that attention operated in this periodic regime, even when it was focused on a single target. That is, attention might rely on an intrinsically periodic process.oscillation ͉ parallel vs. serial S elective attention denotes the ability to enhance processing of a particular location or object. In recent years a number of studies have suggested that multiple locations can be concurrently attended (1-6). In most of these cases, however, it is difficult to distinguish a true (i.e., sustained) division of the attentional spotlight, from a strategy in which a single attentional focus would switch rapidly between the different targets. Indeed, both strategies could explain the occurrence of classic ''set size effects'' (i.e., decreases of performance with increasing number of attended items) either because attention is a limited resource (''parallel'' strategy), or because the effective time that attention samples each object decreases when several objects must be attended (''sampling'' strategy). With respect to visual search tasks, in which a single target must be detected among a variable number of elements, this debate has divided the community for quite some time, with no accepted conclusion [Sternberg S (1973) in Annual Meeting of the Psychonomics Society in St. Louis, MO,. A similarly unresolved argument has been made regarding multiple-object tracking paradigms (15)(16)(17)(18).Here, we propose a quantitative strategy for distinguishing between these alternatives: The psychometric function for detection of a single target as a function of its duration can be used to predict the expected psychometric function for multiple targets, and the predicted shape is quite different for parallel and sequential strategies. We can thus determine which strategy best describes the performance of human observers with multiple attended items. The mathematical details of this method are given in supporting information (SI) Appendix, but the underlying idea can also be understood in simple terms. A ''probe'' event of variable duration must be detected by the observer (Fig. 1). When only a single location is cu...

Spontaneous EEG oscillations reveal periodic sampling of visual attention

“…A recent study by VanRullen et al (8) proposed on the basis of psychophysical models that information could be sampled periodically at a rate of approximately seven items per second, even when only a single location has to be monitored (as in the present study). Moreover, illusory motion reversals in the continuous wagon-wheel illusion, which are thought to be caused by a periodic sampling mechanism (9,13,22), are much decreased when the moving stimuli are not attended (9).…”

“…Transient attention is captured automatically by sudden salient events, whereas sustained attention can be controlled at will. Although the term "sustained" suggests that the facilitative effect of attention endures continuously as long as attention is deployed at a particular location, it is conceivable that attention operates in a periodic rather than a continuous fashion (8,9). This periodicity of attention is implicit in many visual search tasks in which attention is assumed to switch successively between the different display elements (10-12).…”

“…This periodicity of attention is implicit in many visual search tasks in which attention is assumed to switch successively between the different display elements (10)(11)(12). On the basis of modeling of human psychometric functions in a signal detection task, VanRullen et al (8) recently proposed that attention could sample information at a rate of approximately seven items per second-no matter whether attention was focused on multiple targets or on just a single item. A similar attention-driven periodic sampling mechanism (albeit at a different frequency of ≈13 Hz) has been suggested to account for illusory motion reversals in the continuous wagon-wheel illusion (9,13,14).…”

Attention searches nonuniformly in space and in time

“…It has been proposed that attention samples visual stimuli periodically rather than continuously (13)(14)(15)(16)(17)(18). This question is connected to the uniform vs. nonuniform debate in that the nonuniform (or sequential) model of attention processing maps rather naturally onto a periodic sampling of visual information (with the periodicity reflecting the switching between stimuli).…”

From Sensation to Perception: Using Multivariate Classification of Visual Illusions to Identify Neural Correlates of Conscious Awareness in Space and Time

“…However, an important limitation of this paradigm is that trial-by-trial fluctuations in an observer's attentiveness covary with detection performance. These could be random (as in lapses or fatigue) or rhythmic (recent evidence suggests that, in the visual domain, attention samples visual information at a rate of 4-8 Hz; eg VanRullen, Carlson, & Cavanagh, 2007). In either case a contrast of hits and misses is likely to be confounded by attentional mechanisms or the effects of attentional availability.…”